A typical nonvolatile semiconductor memory device such as a NAND flash memory is increasing its application to multimedia cards by its capability to retain information without power supply. Recent demands for larger storage capacity calls for further integration of memory cell transistors. The problem inherent in integration is the increase in the so called Yupin effect. Narrower spacing between the neighboring memory cell transistors leads to increase in parasitic capacitance between the gate electrodes, whereby the transistors are increasingly affected by the write operation performed on the neighboring cells. Erroneous write operations occur more frequently under such conditions.
An insulating film is generally filled between the gate electrode gaps in order to insulate the neighboring gate electrodes. In case only a silicon oxide film is employed as the insulating film for gap fill purposes, unintended oxidizing reaction may occur at the gate electrode by foreign moisture permeating the silicon oxide film during thermal treatment. This may alter the structure of the gate electrode, resulting in failure in obtaining the desired electrical properties. Silicon nitride film is known for effectively preventing moisture permeation. One example of employing silicon nitride film for such purpose is disclosed in JP 2003-297956 A, for example. The use of silicon nitride film poses negative impact on device operation since the silicon nitride film, having higher dielectric constant than silicon oxide film, causes increase in parasitic capacitance as the gaps between the cells become narrower due to device integration.